Stereo Preamplifier With Bass Boost

Description

This preamplifier was designed to cope with CD players, tuners, tape recorders etc., providing an ac voltage gain of 4, in order to drive less sensitive power amplifiers. As modern Hi-Fi home equipment is frequently fitted with small loudspeaker cabinets, the bass frequency range is rather sacrificed. This circuit features also a bass-boost, in order to overcome this problem. You can use a variable resistor to set the bass-boost from 0 to a maximum of +16dB @ 30Hz. If a fixed, maximum boost value is needed, the variable resistor can be omitted and substituted by a switch.

Circuit Diagram:

Parts:

• P1 = 10K
• P2 = 100K
• R1 = 100K
• R2 = 100K
• R3 = 15K
• R4 = 10K
• R5 = 22K
• R6 = 15K
• R7 = 1K
• R8 = 470R
• C1 = 2.2uF-25v
• C2 = 2.2uF-25v
• C3 = 470uF-35v
• C4 = 1uF-35V
• C5 = 2.2uF-25v
• C6 = 47nF-63v
• C7 = 22uF-25v
• IC1 = TL072, Opamp
• SW1 = DPST Switch

Notes:

• Schematic shows left channel only, but R1, R2, R3 and C1, C2, C3 are common to both channels.
• For stereo operation P1, P2 (or SW1), R4, R5, R6, R7, R8 and C4, C5, C6, C7 must be doubled.
• Numbers in parentheses show IC1 right channel pin connections.
• A log type for P2 ensures a more linear regulation of bass-boost.
• Needing a simple boost-in boost-out operation, P2 must be omitted and SW1 added as shown in the diagram.
• For stereo operation SW1 must be a DPST type.
• Please note that, using SW1, the boost is on when the switch is open, and off when the switch is closed.